1. Field of the Invention
The present invention relates to an apparatus and a method for positioning a read/write head for a hard disk drive, and more particularly to using the phenomenon of repeatable run-out to efficiently compensate a servo loop used for positioning the head.
2. Description of the Related Art
In a hard disk drive, positioning a read/write head over a disk medium is accomplished by having a servo loop lock to some predetermined servo positioning pattern. Such a pattern is usually referred to as a servo wedge. Referring to FIG. 1, a typical servo wedge 100 is a data string that contains several data fields, including a sync field 105, a sync-mark-plus-identification field 110, and some number of position error fields 115, 120, 125. The sync field 105 is used to lock the servo timing loop clock phase and frequency to the servo pattern. The sync-mark-plus-identification field 110 indicates both the circumferential position and the coarse radial position of the head. The position error fields 115, 120, 125 provide information concerning the fine radial positioning of the head.
Typically, the read/write head will be positioned by a servo unit to a fixed position as defined information contained in the servo wedges (servo track following). More specifically, the read/write head is mounted on a head arm. The head arm is part of an actuator assembly which conventionally comprises an actuator body or voice coil motor that rotates on a pivot assembly. The head arm is moved by the actuator to a predetermined servo track so that the read/write head is in the proper position to perform a desired operation (read or write). Ideally, the servo tracks are concentric with the spindle motor axis, and there is no force exerted on the actuator assembly when the disk is spinning and when the actuator is not driven. However, it is possible that either the servo track is not written concentrically during the servo write operation, or the disk platter is not exactly perpendicular to the axis of rotation. Because of these two possible contingencies, the actuator current which drives the actuator must be adjusted to xe2x80x9ctrack-followxe2x80x9d even when there are no other external forces or disturbances.
The repeatable portion of these built-in disturbances is commonly known as repeatable run-out (RRO). The RRO phenomenon may be compensated for by analyzing the actuator current during servo track following. The built-in RRO is characterized by dividing the actuator current by the position-to-current transfer function of the servo loop. The run-out values can then be subtracted from the position error signal (PES) to compensate for the built-in RRO disturbance of the disk drive.
The process of characterizing the repeatable run-out values over the circumference of a disk is a fairly long process of averaging many revolutions of servo signals to suppress noise that exists in the system. The RRO values also vary with radial and circumferential position, thus necessitating a large matrix of RRO values that must be stored. If such a matrix is not stored, then the RRO values must constantly be recalculated on the fly; this leads to severe degradation of disk access performance. Conventionally, the RRO information is stored on the disk itself. It has been found that the most efficient storage method is to embed the RRO values within a servo wedge, usually one or two wedges before the RRO values are needed for RRO compensation.
Referring to FIG. 2, a common format for a servo wedge 200 containing RRO data is shown. Up to two RRO fields 205, 210 may be required, because the read process and the write process are performed at different track positions due to a physical offset between the read element and the write element on the read/write head. In some circumstances, however, RRO compensation is performed only for the write process, because in general, repeated attempts to perform the write process are not made, so it is more critical to perform the write process accurately the first time, whereas the read process may be retried if unsuccessful.
The storing of the matrix of RRO values may be performed during the drive characterization within the manufacturing process, or after the original servo wedge has been written. For this reason, the servo channel and/or the hard disk controller (HDC) must be capable of writing the RRO matrix at the end of the servo wedge.
In the conventional methodology for repeatable run-out compensation, as described above, the additional one or two data fields devoted to RRO values have the effect of increasing the total size of the servo wedge, thus causing a reduction in the available disk space for user data, i.e., a loss of formatting efficiency. In recognition of this problem, the present invention provides an apparatus and method for eliminating the inefficiency of data formatting associated with the inclusion of one or more RRO fields without sacrificing accuracy in performing the repeatable run-out compensation.
The present invention is intended to address the need for providing repeatable run-out compensation for a servo in a high-speed read/write channel for a hard disk drive without the inefficiency of using valuable data storage space on a disk medium for sets of repeatable run-out values.
In one aspect, the invention provides an apparatus and a process for positioning a read/write head for a hard disk drive, and more particularly for using the phenomenon of repeatable run-out to efficiently compensate a servo loop used for positioning the head. The apparatus includes a read/write channel and a processor operating in a system that includes a read/write head, a disk medium, and an actuator for positioning the read/write head with respect to the disk medium. The disk medium includes at least one original servo wedge for providing position information to the servo. The original servo wedge includes a sync field, an identification field, and at least one position error field. The identification field includes a sync-mark. The processor is configured to instruct the read/write channel to retrieve the original servo wedge from the disk medium. The processor is also configured to calculate repeatable run-out values using position-error-signal values contained in the original servo wedge. The processor is also configured to use the repeatable run-out values to generate a new servo wedge that contains updated position information so that the actuator current is minimized. The processor is configured to instruct the read/write channel to write the new servo wedge to the disk medium. The new servo wedge may be written to a predetermined location of the disk medium. The predetermined location may be determined based upon a fixed time interval and the sync-mark of the original servo wedge. The original servo wedge may be erased or overwritten with data.
In another aspect, the invention provides a process for positioning a read/write head in a read/write channel for a hard disk drive. The process includes the steps of reading an original servo wedge from a disk medium, commencing to perform servo track following based on the original servo wedge, generating a set of position-error-signal (PES) values from the servo track following, calculating a first set of repeatable run-out values using the PES values, using the first set of repeatable run-out values to compensate a servo loop such that an amount of actuator current is minimized, generating a new servo wedge that includes information updated on the basis of the compensated servo loop, and writing the new servo wedge to the disk medium. The original servo wedge includes a sync field, an identification field, and at least two position error fields. The identification filed includes a sync-mark. The first set of repeatable run-out values is associated with a write element of the read/write head. The new servo wedge may be written to a predetermined location that does not overlap with a location of the original servo wedge. The predetermined location may be determined based upon a fixed time interval and a location of the sync-mark of the original servo wedge. The original servo wedge may be erased or overwritten with data.